Polyphase exhumation of eclogite-bearing high-pressure mica schists from the Fore-Sudetic Block, SW Poland.

Abstract

Mica schists with eclogite lenses appear within the Fore-Sudetic Block in the vicinity of Kamieniec Ząbkowicki. In this paper their metamorphic and structural evolution and geotectonic context is discussed. The mica schists registered a clockwise metamorphic P-T path with two peaks of metamorphism - a pressure peak and a temperature peak. This path indicates compression and crustal thickening during the initial stage of metamorphism, continuing until pressure peak conditions were achieved during continental collision. The minimum pressure and temperature conditions for the pressure peak of these mica schists (11-12 kbar and 400-430°C) and the presence of replacement pseudomorphs after lawsonite indicate that during HP metamorphism the mica schists were at shallower depths than the eclogites and the schists experienced borderline blueschist/eclogite facies conditions. In the mica schists, decompression took place under conditions of rising temperature until the temperature peak was achieved under amphibolite facies conditions (579+/-35°C and 7.4+/-0.2 kbar). This was connected to the upwelling of Variscan granitoid magmas. After the temperature peak of metamorphism further decompression was isothermal. The mica schists were deformed at least four times. The earliest structures (F1) are defined by a HP mineral assemblage (phengite, pseudomorphs after lawsonite, kyanite, rutile, chloritoid, paragonite). The exhumation of the mica schists mostly occurred in a low-angle (?) normal faulting regime (F2) producing SW-vergent structures, coeval with the upwelling of the granitoid magmas. Further uplift and exhumation during isothermal decompression went on by transpression and thrusting to the east (F3) in a continued collisional setting. Late orogenic extension (F4) in the Kamieniec Ząbkowicki area was accomplished by normal faulting on zonally reworked WSW-dipping S2 surfaces.